Testing the engine compression on your 4.3L or 5.0L or 5.7L GM equipped vehicle is not hard. In this article, I'll guide you every step of the way and I'll also show you how to interpret the results of the compression test.
Why do an engine compression test? Well, if you're wanting to know the mechanical condition and/or sealing effectiveness of the piston rings or cylinder head valves, an engine compression test is the only way to ascertain this info.
Not only that, but your 4.3L (or 5.0L, 5.7) car, pick up, mini-van, van or SUV may be suffering a hard to diagnose misfire code that has you pulling your hair out (and maybe even spending money on parts your vehicle doesn't need) and lighting up the check engine light (MIL) with the following misfire diagnostics trouble codes: P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308. If this is happening to you... an engine compression test is in order for your vehicle.
OK, to make it as easy as possible to navigate this article, here are its contents at a quick glance:
You can find this tutorial in Spanish here: Prueba: Compresión del Motor (GM 4.3L, 5.0L, 5.7L) (at: autotecnico-online.com).
The image viewer on the left, has two images that will help you to identify the individual engine cylinders. This will come in super handy in the following test steps.
Before you start, take a look at the whole article and familiarize yourself with all of the steps. Please remember to always think safety first, since you'll be working around a cranking engine.
OK, to get this show on the road, I'll first explain the test steps. At the end of the test steps, you'll find two possible test results that will help you to interpret your specific test results. Let's get started:
If possible, the engine in your vehicle must not be HOT and yet not completely cold. So, if your car or pick up (or mini-van, van, SUV) has bee running for an extended amount of time, let it cool down for about an hour.
Now, if your car doesn't start and you're testing a no start condition, don't worry about having a warm engine for the compression test.
The Fuel System has to be disabled and you can easily do this by simply disconnecting the fuel injector ‘Spider’ assembly. This step is important, so don't skip it.
The Ignition System has to be disabled and this can be accomplished by disconnecting the Ignition Coil or the Ignition Module. This step is also very important.
Now, remove all six (or eight spark plugs) spark plugs. As your taking them out, be careful and don't drop any of them on the floor, or you could cause the spark plug's ceramic insulator to break, and this will cause a misfire!
Thread the engine compression gauge into the spark plug hole for the number 1 engine cylinder. Hand tighten the compression gauge only! Do not use any type of tool to get it tight.
OK, when you're ready, have a helper crank up your pick-up or mini-van, etc. as you observe the needle on the compression tester's gauge. Once the needle on the gauge stops climbing, have him or her stop cranking the engine.
Record this compression reading on a piece of paper. Include the number of the cylinder this reading belongs to. Now repeat steps 1 thru' 6 on the other cylinders.
After testing all cylinders and having written down all of your compression test readings, now you need to interpret the results, for this, turn to the next page...
The idea behind the engine compression test (in the case of a misfire condition or rough idle condition) is to find out if any one engine cylinder is not contributing 100% of its power to the overall engine output. If only one cylinder has lower than normal compression, your GM 4.3L (or 5.0L, 5.7L) equipped vehicle will run rough or cause a misfire code to set and turn on the check engine light.
So then, to wrap things up, the individual cylinder compression readings of each engine cylinder can not vary more than 15%... and this is how you can find out:
Grab a calculator and multiply the highest compression reading that you recorded by .15.
So, let's say that cylinder #4 gave you the highest reading of 170 PSI. Well 170 X .15 gives you 26 (25.5 rounded off).
Now, the next step is to subtract 26 from 170... which gives us 144 PSI.
So then, 144 PSI is the lowest possible compression reading that any one of the rest of the engine cylinders can have. Any compression reading below this.. and that engine cylinder will misfire.
Now, so that this calculation can make more sense to you... let's say that my 4.3L C1500 Pick Up (or Astro or Blazer, etc.) gave me the following compression readings:
The next step is to do the math: 175 x .15= 26, 175-26= 149. So, now I know that cylinder #4 is the one causing the misfire!!
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